It is known from the state of the art to form the vanes of a gas turbine as singlets to be connected to the stator of a gas turbine. Because the vanes are exposed to the hot gases of the gas turbine, for reasons of improving overall performance and efficiency, the vanes have to be coated with coating materials different than the base material from which the vanes are manufactured. It is generally known from the state of the art to use MCrAlY as a bond layer coating to provide a resistance to oxidation and corrosion to the base material. On top of this bond coating, there is applied a second or top coating of yttrium stabilized zirconia having thermal insulating and anti-corrosion properties.
These coatings are typically applied by a plasma spray head. However due to the complex geometry it is very difficult to spray more then one vane at a time. The coating of each vane has the known advantage that it is possible to coat each single vane in the same quality and at a uniform thickness at all the surfaces exposed to the hot gases in comparison to a multi-vane-segment, where due to the geometry only portions can fully be coated to the desired coating thickness and often portions of the vane are only coated with one of both or even remain uncoated. This would immediately lead to undesirable temperature variations along the surface of the vane. For that reason from patent U.S. Pat. No. 5,636,439 a method is known for providing a nozzle segment as a singlet, applying to each singlet a bond and a top layer coating, and then securing two nozzle segments to one another by welding margins of adjacent walls to one another. This is done by butting the margins of the inner walls of adjoining segments against one another whereby the welding includes fusing the material of the butting margins in a direction along that margins terminating shortly before the coated wall surfaces such that said wall surfaces and the base and top coatings there along are not penetrated by fused material. The distance between the coated surfaces and the fused material might be less than 8 mils. The problem with that state of the art occurs due to the fact that during the solidification after welding microcracks are formed in the base material. The microcracks may range from 500 microns to more than 1 millimetre. Such microcracks constitute a zone of such weakness that they can not be allowed in the component, even in moderately loaded areas. Cracks may start growing immediately at this point of weakness. However, the material of choice for a particular vane application is seldom based on its weldability but rather the castability, high temperature strength and environmental resistance. Hence, it will often be the case that the material from which vanes are manufactured is not easily or acceptably weldable. However, the step of welding is desirable because it clearly reduces the leakage of the cooling air through the margins of two adjacent vanes and so the overall efficiency of the gas turbine is increased.